Re: [PATCH v2 07/11] mm: add find_get_entries_tag()

[Dave Chinner <david@xxxxxxxxxxxxx>]

On Fri, Nov 13, 2015 at 05:06:46PM -0700, Ross Zwisler wrote:
> Add find_get_entries_tag() to the family of functions that include
> find_get_entries(), find_get_pages() and find_get_pages_tag().  This is
> needed for DAX dirty page handling because we need a list of both page
> offsets and radix tree entries ('indices' and 'entries' in this function)
> that are marked with the PAGECACHE_TAG_TOWRITE tag.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
>  include/linux/pagemap.h |  3 +++
>  mm/filemap.c            | 61 
> +++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 64 insertions(+)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index a6c78e0..6fea3be 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -354,6 +354,9 @@ unsigned find_get_pages_contig(struct address_space 
> *mapping, pgoff_t start,
>                              unsigned int nr_pages, struct page **pages);
>  unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
>                       int tag, unsigned int nr_pages, struct page **pages);
> +unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
> +                     int tag, unsigned int nr_entries,
> +                     struct page **entries, pgoff_t *indices);
>  
>  struct page *grab_cache_page_write_begin(struct address_space *mapping,
>                       pgoff_t index, unsigned flags);
> diff --git a/mm/filemap.c b/mm/filemap.c
> index d5e94fd..89ab448 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1454,6 +1454,67 @@ repeat:
>  }
>  EXPORT_SYMBOL(find_get_pages_tag);
>  
> +/**
> + * find_get_entries_tag - find and return entries that match @tag
> + * @mapping: the address_space to search
> + * @start:   the starting page cache index
> + * @tag:     the tag index
> + * @nr_entries:      the maximum number of entries
> + * @entries: where the resulting entries are placed
> + * @indices: the cache indices corresponding to the entries in @entries
> + *
> + * Like find_get_entries, except we only return entries which are tagged with
> + * @tag.
> + */
> +unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
> +                     int tag, unsigned int nr_entries,
> +                     struct page **entries, pgoff_t *indices)
> +{
> +     void **slot;
> +     unsigned int ret = 0;
> +     struct radix_tree_iter iter;
> +
> +     if (!nr_entries)
> +             return 0;
> +
> +     rcu_read_lock();
> +restart:
> +     radix_tree_for_each_tagged(slot, &mapping->page_tree,
> +                                &iter, start, tag) {
> +             struct page *page;
> +repeat:
> +             page = radix_tree_deref_slot(slot);
> +             if (unlikely(!page))
> +                     continue;
> +             if (radix_tree_exception(page)) {
> +                     if (radix_tree_deref_retry(page))
> +                             goto restart;

That restart condition looks wrong. ret can be non-zero, but we
start looking from the original start index again, resulting in
duplicates being added to the return arrays...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx